The invention relates to a pulsed doppler radar system comprising a transmitter for transmitting pulse-shaped signals and a receiver having a mixing device to which the transmitter signal and a received transmitter signal reflected from a target are applied, also comprising an evaluation circuit connected to the mixing device.
When measuring apparatus are used for velocity measurements by means of a so-called doppler radar, a transmitter is generally used in which the transmitter signal is radiated in the so-called continuous wave mode. The relatively high current consumption this requires is not important when the required voltage can be obtained from power mains or for a short period of time from a car battery. Completely different requirements must be satisfied when the doppler radar devices must be used for intrusion detection systems as they must then compete with intrusion detection systems which operate on infrared bases. These intrusion detection systems must namely satisfy the requirement that they can be switched over automatically to battery supply in the case of a power cut. The current consumption of such a system is then decisive for the period of time in which performance reliability is still certain.
An intrusion detection system having a short wave doppler radar module, type indication MDX 0724, is disclosed in Valvo Brief "Messeinformationen Hannover" 1978, dated 19.4.1978, page 50 section 1. In conjunction therewith there is a further publication for the unit MDX 0622 for an X-band doppler radar unit. In addition thereto, Messrs. Valvo published a provisional data sheet in March 1978, for the short wave doppler radar module which relates to a compact VHF transmission-receiving unit with a stable, cavity Gunn-oscillator and having a diode mixer and planar transmitting-receiving aerial structure for the construction of motion detectors. The module described there has a range of 15 m and an operating current of 150 mA. The operating frequency is 13.55 GHz.
The 150 mA operating current for the module MDX 0524 was high and therefore a doppler radar module MDX 0724, which operates in the pulse mode and has a current of 15 mA was already proposed in the publication Valvo Brief dated 19.4.1978 on the Hannover Fair 1978. Only the HF portion with the control circuit was shown on the Hannover Fair. The evaluation circuit was lacking and the present invention relates to a portion of this evaluation circuit arrangement.
On the basis of the above prior art it was known to operate a doppler radar in the GHz range with a keyed transmitter oscillator. When a simple Gunn oscillator is keyed by means of its operating voltage, its frequency changes during the keying pulses as the result of the fact that the HF oscillator is warmed up during each pulse. A requirement was that the doppler signal frequency must yet be separated from the keying frequency. For the known devices, which operate at approximately 10 GHz the doppler signal frequency amounts up to 100 Hz and when the doppler signal must still be visible with a keyed transmitter, the keying frequency must be higher than 200 Hz. The invention had therefore for its object to take all these known problems into consideration and to propose a switching arrangement in which a certain optimization is obtained.